Compounds having low ionization energy

ABSTRACT

The present invention provides compounds that are soluble in a non-polar solvent and having a low ionization energy and negative oxidation potentials in tetrahydrofuran (THF). The present invention also provides a method for producing and using the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. ProvisionalApplication No. 61/880,135, filed Sep. 19, 2013, which is incorporatedherein by reference in its entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH

This invention was made with government support under grant numberCHE-1111570 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a compound that is soluble in anon-polar solvent and/or having a low ionization energy, negativeoxidation potentials in tetrahydrofuran (THF, e.g., versusferrocene/ferrocenium), or both. The present invention also relates to amethod for producing and using the same.

BACKGROUND OF THE INVENTION

The phenomena of oxidation (electron loss) and reduction (electron gain)are fundamental to chemistry. Even when these processes do not occur assuch, more complex chemical processes and properties, such as ionic andcovalent bond formation and acid-base behavior, can be understood byincluding them as a sum of oxidation and/or reduction steps. Tables ofoxidation enthalpies, electron affinities, and electrode (redox)potentials are essential to understanding and teaching chemistry

Stable, strong oxidizing and reducing agents have many importantchemical and materials applications. Especially needed are strong redoxagents to be utilized in non-aqueous, homogeneous systems. While strongredox agents have recently been developed, including by the presentinventors, many of these compounds are not soluble in nonpolar and/oraprotic solvents, thereby rendering them virtually useless in chemicalreactions requiring nonpolar and/or aprotic solvent.

Therefore, there is a need for strong redox agents that are soluble innonpolar and/or aprotic solvents.

SUMMARY OF THE INVENTION

Some aspects of the invention provide compounds comprising a metal andan organic ligand. In some embodiments, the compounds of the inventionhave a low ionization energy and negative oxidation potential on theelectrochemical scale. Therefore, compounds of the invention are usefulin a wide variety of applications including, but not limited to, asreducing agents, catalysts, in solar cells, in electronics (such ascapacitors, light emitting diodes, etc.), in nanoparticles, etc. Thecompounds of the invention can also be used in hydrogen production, aswell as catalysts in solar fuel production. In general, compounds of theinvention can be used in any applications that utilize a compound havinga low ionization energy and negative oxidation potential.

In some embodiments, the onset ionization energy of the compound of theinvention is about 4 eV or less, typically 3.8 eV or less, often 3.6 eVor less, and more often 3.5 eV or less.

Yet in other embodiments compounds of the invention are soluble in anaprotic organic solvent. Exemplary aprotic organic solvents include, butare not limited to, diethyl ether, tetrahydrofuran (THF),dimethylsulfoxide (DMSO), benzene, toluene, methylene chloride,chloroform, acetonitrile, ethyl acetate, acetone, hexane, pentane,1,4-dioxane, dimethylformamide (DMF), etc.

Still in other embodiments, compounds of the invention are soluble in anonpolar organic solvent. As used herein, the term “nonpolar” refers toa solvent having a dielectric constant of about 10 or less, typically 8or less, often 7 or less, and more often 5 or less. Exemplary nonpolarorganic solvents include, but are not limited to, pentane, hexane,cyclohexane, benzene, toluene, 1,4-dioxane, chloroform, diethyl ether,etc.

Yet in other embodiments, compounds of the invention have an oxidationpotential in THF of about 0.0 V or less, typically −1.5 V or less, often−1.8 V or less versus ferrocene/ferrocenium.

The solubility of compounds of the invention in THF is at least 0.1 g/L,typically at least 0.5 g/L, often at least 1 g/L, more often at least 2g/L, and most often at least 5 g/L. The solubility of compounds of theinvention are typically determined under standard conditions, e.g., 20°C. at 1 atmosphere of pressure.

In some embodiments, the solubility of compounds of the invention inbenzene is at least 0.1 g/L, typically at least 0.5 g/L, often at least1 g/L, more often at least 2 g/L, and most often at least 5 g/L.

In some embodiments of the invention, the compound of the invention is atungsten compound of the formula:W₂(L)₄X_(m)where L is an anionic organic ligand, X is a halide, and m is an integerfrom 0 to 2. As used herein, unless the context requires otherwise theterms “anionic organic ligand” and “organic ligand” are usedinterchangeably herein and refer to a ligand that compriseshydrocarbons. Hydrocarbon can be saturated, unsaturated, linear, cyclic(e.g., mono-cyclic, bicyclic, tricyclic, etc.) or a combination thereof.The organic ligand can also include a heteroatom selected from the groupconsisting of N, O, S, or P. In some embodiments, the anionic organicligand is a bicyclic heterocycloalkyl. Yet in other embodiments, theanionic organic ligand is a bicyclic heterocycloalkyl comprising one tofive, typically one to four, often one to three heteroatom that isindependently selected from the group consisting of N, O, S, and P.

Still in other embodiments, each L is nitrogen atom containing bicyclicheteroalkyl, where the number of nitrogen atom in the cyclic ringstructure ranges from one to five, typically one to four, and often oneto three. In one particular embodiment, each L is independently alkylsubstituted guanidinate. As used herein, the term “alkyl” refers to asaturated linear monovalent hydrocarbon moiety or a saturated branchedor cyclic monovalent hydrocarbon moiety. Optionally, one or more of thehydrogen atom in the alkyl group may be replaced with a halide such aschloro, bromo, fluoro or iodo. Furthermore, one or more of the carbonatom in the alkyl chain may be replaced with a heteroatom such as O, N,S or P. Suitable alkyl substituents for guanidinate include C₁-C₂₀alkyl, typically C₁-C₁₂ alkyl, often C₁-C₈ alkyl, and more often C₁-C₄alkyl. Typically, each L has at least one alkyl substituent, typicallyat least two alkyl substituents, often at least three alkylsubstituents, and more often at least four alkyl substituents. Exemplaryalkyl substituents include, but are not limited to, methyl, ethyl,propyl, isopropyl, butyl, sec-butyl, isobutyl, tert-butyl, pentyl,methoxymethyl, methoxyethyl, trifluoromethyl, trichloromethyl,chloromethyl, and the like. It should be appreciated that in someinstances, L can be a chiral ligand, and therefore, the scope of theinvention also includes enantiomerically enriched compound (i.e., acompound having an enantiomerically enriched L group(s)). As usedherein, the term “guanidinate” refers to a moiety having1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine structure and itsderivatives.

In one particular embodiment of the invention, the compound of theinvention is W₂L₄.

Still in other embodiments, each L is independently a moiety of theformula:

where each of m and n is independently an integer from 0 to 4, typically0 to 3, provided the sum of m and n is at least 1, typically at least 2,often at least 3, and more often at least 4; each of R¹ and R² isindependently alkyl, typically C₁-C₁₂ alkyl, often C₁-C₁₀ alkyl, moreoften C₁-C₈ alkyl, and most often C₁-C₆ alkyl. In one particularinstance, n is 4. Still in other instances, each of R¹ and R² isindependently methyl, ethyl, or C₃ or C₄ alkyl. In one particularembodiment, m and n are 2. Still in other embodiment, each of R¹ and R²is methyl or ethyl.

Yet in other embodiments, compounds of the invention are thermallystable and easy to synthesize in high yields and good purity. They arevery reactive and potentially useful stoichiometric reducing agents innon-polar, non-protonated solvents.

Still further, combinations of the various embodiments described hereinform other embodiments. For example, in one particularly embodiment R¹and R² are ethyl, and m and n are 2. In this manner, a variety ofcompounds are embodied within the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photoelectron spectra of the low ionization energy regions(δ bond ionizations) of W₂(bicyclic guanidinate)₄ complexes.

FIG. 2 is a differential pulse voltammograms in THF for W₂Cl₂(TMhpp)₄(solid line) and W₂Cl₂(TEhpp)₄ (broken line).

FIG. 3 is the representative structures of compound 1.4CH₂Cl₂ on theright and compound 3 on the left with displacement ellipsoids drawn atthe 30% probability level. Solvent molecules in compound 1 and allhydrogen atoms have been removed for clarity.

FIG. 4 is an optimized representative structure of W₂(hpp)₄(THF)₂ ⁺shown with van der Waals atomic radii. The van der Waals contacts impedeclose association of the THF solvent molecules with the W₂ core. Colorcodes: orange=tungsten; red=oxygen; blue=nitrogen; black=carbon andgray=hydrogen atoms.

DETAILED DESCRIPTION OF THE INVENTION

Stable, strong oxidizing and reducing agents have many importantchemical and materials applications. Especially needed are strong redoxagents that are soluble in non-aqueous, nonpolar or aprotic solvent. Asused herein, the term “soluble” refers to a compound that has solubilityof at least 0.1 g/L, typically at least 0.25 g/L, often at least 0.5 g/Lmore often at least 1 g/L, still more often at least 2 g/L, and mostoften at least 5 g/L in THF. Thus, compounds of the invention allowhomogeneous systems for carrying out a chemical reaction in a non-polarand/or aprotic organic solvent.

Some aspects of the invention are based on the discovery by the presentinventors that compounds with metal-metal bonds with an anionic organicligand (e.g., a bridging bicyclic guanidinate ligand) can oftenstabilize dimetal units in high oxidation states. In some cases, thecommon M₂ ⁴⁺ units (where each M is independently a metal selected fromthe group consisting of tungsten or chromium) become extremelythermodynamically unstable to oxidation, the prime example beingW₂(hpp)₄ (see below for ligand representations for “hpp”, “TMhpp” and“TEhpp”). With an onset ionization energy of only 3.51 eV, thisquadruple bonded W₂(hpp)₄ compound has the lowest known ionizationenergy for any molecule prepared in a synthesis laboratory, being evenlower than that of cesium, the most easily ionized stable element.

The redox properties of W₂(hpp)₄ and its ability to act as a reducingagent were investigated generating mixed results. This compound wasfound to be very reactive as a reductant, but a major drawback was thepoor solubility in non-reactive organic solvents. Some aspects of theinvention are based on the discovery by the present inventors that thederivative of hpp ditungsten compound has a significantly highersolubility in non-polar and/or aprotic organic solvent. Thus, by addingalkyl substituents to the hpp backbone, the present inventors havediscovered that the resulting compounds have lower ionization energiesand induce shifts in the negative direction in the oxidation potentialsas well as increased solubility.

As described above, compounds of the invention have a low ionizationenergy and/or negative oxidation potentials. Furthermore, compounds ofthe invention are soluble in non-polar and aprotic solvents such astetrahydrofuran (THF), benzene and other nonpolar solvents. Some of thecompounds of the invention will now be described with reference tosynthesis and characterization of a compound with triple bonded W₂ ⁶⁺cores, namely W₂(TMhpp)₄Cl₂, 1, and W₂(TEhpp)₄Cl₂, 2, that serve asprecursors for the syntheses of W₂(TMhpp)₄, 3, and W₂(TEhpp)₄, 4,respectively. Compounds 1-3 have been characterized by X-raycrystallography and either electrochemical properties, photoelectronspectroscopy (PES), or other spectroscopic and spectrometric techniquesand compound 4 has been characterized by ¹H NMR and PES. The moleculeshave excellent properties for strong reducing agents in organicsolvents.

The photoelectron spectra (FIG. 1) of the first ionization bands ofcompounds 3 and 4 revealed that the ionization energies (“IE”) varyslightly but are lower than the ionization energy of the parent compoundW₂(hpp)₄. The onset ionization energies (IE_(onset)) for the parentcompound is 3.51±0.05. IE_(onset) for compounds 3 and 4 was determinedto be 3.45±0.03 and 3.40±0.05 eV, respectively, making compounds 3 and 4as compounds with the lowest IEs known to date. For comparison, the IEfor cesium is 3.89 eV. The IE for 3 was measured in the temperaturerange from 216 to 271° C. over a period of 2½ h while that for 4 wasmeasured in the temperature range of 317-340° C. for about 1 h. Inneither case was there any evidence of decomposition. This indicatesthat the thermal stability of both compounds in a vacuum is very high.As solids these species are almost indefinitely stable when stored insealed ampoules protected from air and from protonated or halogenatedsolvents.

Electrochemical measurements in THF of compounds 1 and 2 showed thateach compound has two reversible redox waves at negative potentials. Inthe differential pulse voltammograms (DPVs, FIG. 2), the peaks at −1.84and −1.90 V vs. Ag/AgCl have been assigned to the W₂ ^(5+/4+) processesfor the methyl and ethyl derivatives, respectively, and those at −0.99and −0.99 V correspond to the W₂ ^(6+/5+) processes. As a comparison,the reported E_(1/2) of compound W₂(hpp)₄ is −0.97 V and −1.81 V for W₂^(6+/5+) and W₂ ^(5+/4+), respectively. The shifts in the solutionoxidation potentials for these molecules are the same as the shifts ofthe onset gas phase IEs within experimental uncertainty, indicating thatthe solvation and thermodynamic effects on the relation between theionization energies and oxidation potentials are similar for thesemolecules.

As shown in Scheme 1 below, compounds of the invention can be easilyprepared in good yields using standard Schlenk-type techniques byreaction of commercially available and stable W(CO)₆ with the neutralbicyclic guanidinates in refluxing o-dichlorobenzene. The chlorinatedsolvent is useful not only in providing a high reflux temperature butalso to serve as the oxidizing agent and be the source of the chlorineatoms in 1 and 2. Both compounds 1 and 2 can be handled in dry air overhours when they are in crystalline form. This makes them convenientsources for weighing and general handling of the compounds previous tofurther reactions. Compounds 1 and 2 can be readily reduced to compounds3 and 4, respectively, for example, by treating with excess potassiummetal in refluxing THF. The excess potassium metal can be removed byfiltration, and the THF can be removed to obtain compounds 3 and 4.Removal of the THF solvent provides essentially a quantitative yield ofthe products. Further purification can easily be accomplished byremoving the THF solvent followed by extraction with benzene or toluene.

The present inventors have discovered that compounds 3 and 4 are verysoluble in common dry solvents such as THF, toluene, and benzene. Somesolubility of compound 3 in hexanes is also observed while compound 4 issignificantly more soluble in the latter. In general, as the chainlength of the alkyl substituent in the guanidinate increases, the higherthe solubility in non-polar and/or aprotic solvent was observed.

The solubility properties of compounds 3 and 4 compare very favorably tothose of decamethylcobaltocene, a commonly used reducing agent, but itis worth noting that the IEs and electrode potentials of compounds 3 and4 are significantly more favorable than those of (Cp*)₂Co. Theionization energy of (Cp*)₂Co at 4.705 eV is more than 1 eV greater thanthat of compounds 3 and 4, and the oxidation potential of (Cp*)₂Co at−1.47 V is approximately 0.4 V less negative than that of compounds 3and 4.

The structures of compounds 1 and 3 (FIG. 3) show a paddlewheelstructure with four bicyclic guanidinate ligands spanning the ditungstenunit. For compounds 1 and 2, it appears the chlorine atoms occupy axialpositions at distances of about 2.98 Å and 2.85 Å, respectively. The2.98 Å distance is the longest of over 3500 W—Cl distances found in theCCDC, and far beyond the mean of 2.42 Å, indicating only weak W—Clbonding. Without being bound by any theory, the weakness of this bond isbelieved to be a contributing factor to the electrochemical propertiesof compounds of the invention.

Photoelectron spectroscopy showed that ditungsten tetraguanidinatemolecules of the invention give up electrons at extremely low ionizationenergies, e.g., around 3.4-3.5 eV. A closer examination of theconnection between the ionization energies and the reduction potentialsgenerally provides a better understanding of the strong reducing abilityof these dimetal tetraguanidinate molecules in nonpolar organicsolvents. Scheme 2 shows the connection starting from the experimentalgas-phase vertical ionization energy (VIE or IE_(vertical)) of W₂(hpp)₄shown in blue (i.e., VIE of 3.76) at the top and proceeds to theexperimental W₂ ^(5+/4+) potential measured in THF shown also in blue atthe bottom. In contrast to the gas-phase spectroscopic energy (VIE) thatis measured on a fast timescale, the solution potential measures anequilibrium (free energy) that involves the vibrational and thermalenthalpies and entropies of solvated species. These contributions areshown in green in the diagram as obtained from DFT computations (seeExperimental Section) where E_(reorg) is the geometric reorganizationenergy of the positive ion from the structure of the neutral molecule,ΔG(ν,T) includes the differences in zero-point vibrational energies andtemperature-dependent H(T)−TS(T) contributions to the free energy atstandard temperature and pressure, and ΔG_(solv) is the solvationstabilization energies of the neutral and cationic species.

The sum of these contributions gives the absolute free energy change forW₂(hpp)₄/W₂(hpp)₄ ⁺(W₂ ^(4+/)5+) in THF of 2.85 eV. A similarcalculation for the ferrocene/ferrocene⁺ couple (Fc/Fc⁺) gives anabsolute free energy change of 5.12 eV, so that the ΔG for W₂ ^(4+/5+)is 2.27 eV less than that of Fc/Fc⁺. Since ΔG=−nFE and F=1 for theconversion from eV to V, E_(1/2) for W₂ ^(4+/5+) in THF is calculated tobe −2.27 V vs. Fc/Fc⁺. E_(1/2) for the Fc/Fc⁺ couple consistentlyoccurred at +440 mV vs. Ag/AgCl, so the calculated potential forW₂(hpp)₄/W₂(hpp)₄ ⁺ vs. Ag/AgCl is −1.83 V. This compares very closelyto the observed peak position of −1.81 V measured in THF for W₂(hpp)₄ ²⁺(TFPB⁻)₂ (TFPB⁻=tetrakis[3,5-bis(trifluoromethyl)phenyl]borate).

It should be appreciated that the solution reduction potential isdetermined primarily by two factors. First is the gas-phase verticalionization energy and second is the solvation stabilization of thepositive ion. Because the overall structure of the molecule is largelydetermined by the structure of the bicyclic guanidinate ligands andtheir coordination to the metals, and the structure changes little fromthe neutral to the positive ion, the sum of the reorganization energyand vibrational/thermal contributions shifts the free energy by onlyabout 0.2 eV. Errors in the modeling of these contributions are a smallfraction of this number and have little effect on the calculatedpotential. The good agreement between the calculated and the observedreduction potential then follows primarily from starting with anexperimental gas-phase VIE and having a model that gives good account ofthe ΔΔG_(solv). Nearly all of the models tested were adequate in thisregard for these compounds.

The ditungsten tetraguanidinate complexes in the electrochemical studieshave chloride atoms coordinated to the tungsten atoms rather than TFPB⁻counter ions. Nonetheless the reduction peaks in THF occur at verysimilar potentials to those of W₂(hpp)₄ ²⁺(TFPB⁻)₂. A question is theextent to which these dichloro complexes are soluted to ions in THF. TheW—Cl interaction is weak as evidenced by the long distance, but thecomputations indicate that the W—Cl bond is only about one-thirdelectrostatic. Scheme 3 shows the calculated equilibria for dissolutionof W₂(hpp)₄Cl₂ into ions in THF, along with calculated reductionpotentials for various species. The computations indicate that theneutral dichloro complexes are not appreciably soluted to ions in THF,but reduction increasingly favors dissociation of Cl⁻ from the complex.It is believed that a driving force is the solvation energy of the Cl⁻ion in THF (ΔG_(solv) literature 2.81 eV, DFT calc. 2.91 eV), which alsoshifts the reduction potential less negative. The DFT calculationsestimate the reduction potentials for these complexes too negative andmay overestimate the W—Cl bond strength. But even with theseoverestimates, K in THF (E_(1/2) literature −2.66 V, DFT calc. −2.64 V)will result in the reduction of W₂(hpp)₄Cl₂ to W₂(hpp)₄+2Cl⁻ in THF. Theprimary driving forces are believed to be the solvation stabilization ofthe Cl⁻ ions in comparison to the weak W—Cl interactions and the solventstabilization of the K⁺ ions in comparison to the weak stabilization ofthe W₂(hpp)₄ cations in THF.

One of the possible reasons for the comparatively weak stabilization ofthe W₂(hpp)₄ ⁺ cation in THF, in addition to the size of the molecule,is illustrated in FIG. 4, which shows the optimized structure of theW₂(hpp)₄ monocation with THF molecules in the axial sites. It isbelieved that the van der Waals sizes of the hpp ligands impede the THFoxygen atoms from coming within van der Waals contact with the tungstenatoms, thereby stabilizing the positive charge. In contrast the K⁺ ionhas the advantage of receiving the full stabilization from the THFsolvent.

Because the compounds of the invention have the advantage of beingsoluble in non-polar organic solvents, it is interesting to compare thereducing power of these compounds in non-polar solvents with thehypothetical case of potassium metal in a non-polar solvent. Scheme 4compares the favorable reduction directions in THF and hexane accordingto the free energies obtained from this computational model. Theequilibrium switches direction from THF to hexane, such that in hexaneW₂(hpp)₄ reduces K⁺ to K. Also shown in Scheme 4 is the reducing powerof W₂(hpp)₄ in comparison to decamethylcobaltocene, a commonly-usedstrong reducing agent for stoichiometric reactions. As can be seen,W₂(hpp)₄ is a much stronger reducing agent than decamethylcobaltocene.

Analogues of the most easily ionized molecule have been prepared in goodyields. The ditungsten compounds with four bridging bicyclic guanidinateligands have very low ionization energies and very negative oxidationpotentials. Compounds 3 and 4 are thermally stable and very soluble andstable in non-halogenated, non-protonated solvents such as THF, toluene,benzene and even hexanes. These properties make them ideal candidatesfor use as strong reducing agents and other wide variety ofapplications.

Some of the applications of compounds of the invention in the form ofcatalysts include, but are not limited to, catalysts for hydrocracking,dehalogenation, hydrodesulphurization (HDS), hydrodenitrogenation (HDN)and hydroderomatisation (HAD) of mineral oil products, where tungstenand nickel oxides on ceramic carriers are used; selective catalyticreduction of nitrogen oxides (NOx) (e.g., as catalysts for the removalof nitrogen oxides from stack gases of combustion power plants, chemicalplants, cement plants or diesel engines by selective catalytic reductionwith ammonia or urea; various applications in the chemical industry, forexample dehydrogenation, isomerization, polymerization, reforming,hydration and dehydration, hydroxylation and epoxidation.

Oil, and the gases associated with it, consists of a mixture of hundredsof different hydrocarbons, most of these are straight chain, andsaturated hydrocarbons that have little direct use in the chemicalindustry or as fuel for cars. Thus the various fractions obtained fromthe distillation of crude oil and the associated gases have to betreated further, sometimes through catalytic reactions in oilrefineries, to make them useful. Methods which compounds of theinvention, in particular tungsten containing organometallic compounds,provide a high value proposition are in the following exemplary areas:dehydrogenation, aromatization, catalytic reforming

Dehydrogenation is a chemical reaction that involves the removal ofhydrogen from a molecule. It is the reverse process of hydrogenation.Dehydrogenation processes are used extensively in the oil and gasindustry creating, for example, (i) aromatic compounds from raw crudeoil to create products, e.g., for increasing fuel octane; and (ii)propylene from the decomposition of propane, which can then be used toproduce such organic chemicals as acetone and propylene glycol.

Aromatization is the dehydrogenation and aromatization of paraffins toaromatics. This is the process of converting paraffins obtained from thepetroleum distillation process into valuable aromatics. Aromaticscreated from paraffins can be used to produce high octane gasoline andother refined petro-chemical products.

Catalytic reforming is a chemical process used to convert petroleumrefinery naphthals distilled from crude oil (typically having low octaneratings) into high-octane liquid products called reformates, which arepremium blending stocks for high-octane gasoline. A large number ofreactions occur in catalytic reforming over bifunctional catalysts.Tungsten is used in such reactions as dehydrogenation anddehydroisomerization of naphthenes to aromatics, dehydrogenation ofparaffins to olefins, dehydrocyclization of paraffins and olefins toaromatics. Benzene, toluene, xylene and/or ethylbenzene are produced byreformate, the main source of aromatic bulk chemicals. These aromaticcompounds are importantly as raw materials for conversion into plasticsand other chemicals.

Compounds of the invention can also be used in manufacturing of styrenemonomer from dehydrogenation. The conventional method for producingstyrene involves two steps: the alkylation of benzene with ethylene toproduce ethyl benzene followed by dehydrogenation of the ethyl benzeneto produce styrene. Compounds of the invention can be used as catalystsin the dehydrogenation process to produce, for example, styrene whichcan be used in the fine chemicals, oleochemicals, petrochemicals, anddetergents industries. Styrene is used predominately in the productionof polystyrene plastics and resins. Styrene is also used as anintermediate in the synthesis of materials used for ion exchange resinsand to produce copolymers.

Compounds of the invention can also be used as a catalyst inmanufacturing of surfactants in dehydrogenation reaction. Manysurfactants are produced through dehydrogenation of linear alkylbenzene.For example, these surfactants are often produced by dehydrogenation ofn-paraffins to n-olefins followed by benzene alkylation to producelinear alkylbenzene. The resulting surfactants have improvedbiodegradability and cost-effectiveness compared to other method ofsurfactant production.

Compounds of the invention can also be incorporated into thin films. Oneuse is as an n-dopant in electronic and optoelectronic devices. Tungstenis one of a variety of materials known as refractory metals that diffuseslowly, are able to withstand high temperatures, are relativelynon-reactive, have low resistance and can be processed to yield thinfilms on many different substrates. These properties make tungstencontaining compounds very useful in the manufacturing of today's highperformance integrated circuits (ICs). Physical properties of interestin tungsten films used in semiconductor devices include crystalstructure, phase, surface roughness, composition, stress, grain size,crystal orientation, thickness, and adhesion. Most of these physicalproperties have a direct bearing on the electrical behavior of thetungsten and thus profoundly influence chip performance. In the last twodecades, as processor speeds have risen and feature size onsemiconductor devices has dropped, some of the more conventionalmaterials used in IC processing have become inadequate for the demandsplaced upon them. Aluminum is a commonly used material in ICs but hasdifficulty performing well in new applications. Tungsten is currentlymost commonly used as a contact material, a conductive diffusionbarrier, or as an interconnect between two adjacent metal layers, mostcommonly called a via.

As discussed herein, compounds of the invention have low ionizationenergies, and therefore are strong chemical reducing agents. Some of theparticular applications of compounds having a low ionization energyinclude, but are not limited to, emulsion polymerization, vat dyeing,textile stripping and clearing, ground wood bleaching, recycled paperde-inking, precious metal recovery, waste water treatment and ironreduction. Some of the specifically useful applications of compounds ofthe invention include reduction of nitrogen to ammonia and the reductionof protons to molecular hydrogen. These latter processes are estimatedto consume up to 5% of the world's natural gas, require large amounts ofenergy (e.g., for high temperature and pressure of the reaction), andproduce as a side product hundreds of millions of tons of carbondioxide. Because compounds of the invention are strong reductioncatalysts, using compounds of the invention in these processes caneliminate a significant amount of these problems.

Additional objects, advantages, and novel features of this inventionwill become apparent to those skilled in the art upon examination of thefollowing examples thereof, which are not intended to be limiting. Inthe Examples, procedures that are constructively reduced to practice aredescribed in the present tense, and procedures that have been carriedout in the laboratory are set forth in the past tense.

EXAMPLES Abbreviations

The following abbreviations are used: DFT, Density Functional Theory;PES, Photoelectron Spectroscopy; DPV, Differential Pulse Voltammetry;hpp, the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine;TMhpp, the anion of3,3,9,9-tetramethyl-1,5,7-triazabicyclo[4.4.0]dec-4-ene; TEhpp=the anionof 3,3,9,9-tetramethyl-1,5,7-triazabicyclo[4.4.0]dec-4-ene; VIE=verticalionization energy.

Methods and Materials.

All syntheses were carried out under N₂ using a Schlenk line equippedwith a bubbler with a tube diameter of 2.54 cm and a column of ca. 5 cmof Hg. All manipulations preceding spectroscopic measurements wereperformed under Argon in a glove box. Commercial solvents were treatedas follows: acetonitrile was twice distilled under N₂, first fromactivated molecular sieves and then from CaH₂; THF was distilled fromNa/K benzophenone; dichloromethane was dried and distilled from P₄O₁₀;toluene and isomeric hexanes were dried and degassed using a GlassContour solvent purification system; o-dichlorobenzene was dried overfreshly activated molecular sieves and degassed using vigorous bubblingof N₂ immediately before use. Tungsten hexacarbonyl, was purchased fromcommercial sources; HTMhpp and HTEhpp were prepared according to theliterature procedures. See, for example, Polyhedron 2013, 58, 7-12 andJ. Chem. Phys. B 2006, 110, 19793-19798.

Instrumentation and Characterization.

¹H NMR spectra were recorded on a Mercury 300 spectrometer with chemicalshifts referenced to the protonated solvent residue. Mass spectrometrydata (electrospray ionization, ESI) were recorded at the Laboratory forBiological Mass Spectrometry at Texas A&M University using an MDS SeriesQstar Pulsar with a spray voltage of 5 kV. Elemental analyses wereperformed by Robertson Microlit Laboratories, Inc., Madison, N.J. oncrystalline samples that were previously washed with cold hexanes anddried overnight under vacuum. Infrared spectra were recorded in aPerkin-Elmer 16PC FT IR spectrophotometer using KBr pellets. Electronicspectra were recorded on a Shimadzu UV-2501 PC spectrophotometer. Thedifferential pulse voltammograms were collected using a CH Instrumentsmodel-CHI620A electrochemical analyzer in a 0.1 M Bu^(n) ₄NPF₆ solutionin THF, using Pt working and auxiliary electrodes, a Ag/AgCl referenceelectrode, and a scan rate of 100 mV/s. All potential values arereferenced to the Ag/AgCl electrode, and under the present experimentalconditions, the E_(1/2) for the Fc⁺/Fc couple consistently occurred at+440 mV.

W₂(TMhpp)₄Cl₂, 1.

A mixture of 0.240 g (0.682 mmol) of W(CO)₆ and 0.300 g (1.53 mmol) ofHTMhpp was placed in an oven-dried 100 mL Schlenk flask equipped with astir bar, and filled with nitrogen. An aliquot of 15 mL of dried andoxygen-free o-dichlorobenzene was then added, and the flask was fittedwith a previously oven-dried water-cooled cold finger. The pale yellowreaction mixture was refluxed at 210° C. under nitrogen for 6-8 h. Thesolvent was removed by pumping under vacuum at 70° C. The greenish-brownsolid was extracted with 50 mL of toluene, and the mixture filteredunder nitrogen using an oven-dried fritted-glass packed with Celite. Thesolvent from the green-brown solution was removed under vacuum and thesolid washed with hexanes. The solid was dissolved in 10 mL ofdichloromethane and the solution was carefully layered with 40 mL ofhexanes using a 60 mL Schlenk tube and a glass cap that was wrapped withParafilm. After four weeks, dark green-brown, block-shaped crystalssuitable for X-ray diffraction were obtained. Isolated yield 0.378 g,91%. ¹H NMR: (C₆D₆, 300 MHz, 25° C.): δ_(H)=4.17 ppm (s, 16H, 8CH₂),2.63 ppm (s, 16H, 8CH₂), and 1.07 ppm (s, 48H, 16CH₃). IR (KBr, cm⁻¹):ν=2958, 1639, 1530, 1399, 1277, 1125 and 778. UV-vis: λ_(max)=349.5 nm.ESI-MS: m/z=1179.5, [W₂(TMhpp)₄Cl]⁺; m/z=572.25, [W₂(TMhpp)₄]²⁺.Electrochemistry in THF vs Ag/AgCl: E_(1/2)(1)_(THF)=−0.99 V,E_(1/2)(2)_(THF)=−1.84 V. Elemental microanalysis calcd. forC₄₄H₈₀Cl₂N₁₂W₂: C, 43.46; H, 6.63; N, 13.82. found: C, 43.40; H, 6.88;N, 13.64. Crystallographic Data for 1.4CH₂Cl₂: M_(r)=1555.50,orthorhombic, Pcca, a=24.256(5), b=23.817(5), c=25.791(5) Å, V=14,900(5)Å³, Z=8, ρ_(c)=1.387 Mg m⁻³, T=213(2) K, λ=0.71073 Å. 92163 reflectionswere collected, 16,921 independent [R(int)=0.0371], which were used inall calculations. R₁=0.0270, wR₂=0.0609 for observed unique reflections[F²>2σ(F²)] and R₁=0.0631, wR2=0.0677 for all unique reflections. Max.and min. residual electron densities 1.573 and −2.422 eÅ⁻³.

W₂(TEhpp)₄Cl₂, 2.

This compound was prepared similarly to 1 using a mixture of 0.240 g(0.682 mmol) of W₂(CO)₆ and 0.500 g (1.42 mmol) of HTEhpp in 15 mL ofo-dichlorobenzene. After refluxing and removal of the solvent thegreen-brown solid was extracted with 50 mL of a mixture 4:1hexanes/toluene and then the mixture was filtered under nitrogen in anoven-dried fitted glass charged with Celite. After the solvent wasremoved under vacuum from the filtrate, the solid was covered overnightwith hexanes at −20° C., and then the supernatant liquid was removedusing a cannula. The solid was dissolved in 20 mL of a 4:1hexanes:toluene mixture. The tube was placed in a refrigerator at −30°C. After four weeks dark green-brown, block-shaped crystals suitable forX-ray diffraction were collected. Isolated yield 0.443 g, 90%. ¹H NMR:(C₆D₆, 300 MHz, 25° C.): δ_(H)=4.10 pm (s, 16H, 8CH₂), 2.81 ppm (s, 16H,8CH₂), 1.49 ppm (q, 32H, 16CH₂) and 0.91 ppm. (s, 48H, 16CH₃). IR (KBr,cm⁻¹): ν=2961, 1637, 1534, 1380, 1275, 1123 and 807. UV-vis:λ_(max)=347.5 nm. ESI-MS: m/z=1403.7, [W₂(TEhpp)₄Cl]⁺; m/z=684.4,[W₂(TEhpp)₄]²⁺. Electrochemistry in THF vs Ag/AgCl: E_(1/2)(1)=−0.99 V,E_(1/2)(2)=−1.90 V. Elemental microanalysis calcd. for C₆₀H₁₁₂Cl₂N₁₂W₂:C, 50.03; H, 7.83; N=11.67. found C, 49.83; H=7.69, N, 11.46.Crystallographic Data for 2: M_(r)=1440.22, monoclinic, P2₁/c,a=16.894(4), b=16.778(4), c=22.935(6) Å, β=97.635(5), V=6443(3) Å³, Z=4,ρ_(c)=1.485 Mg m⁻³, T=213(2) K, λ=0.71073 Å. 42958 reflectionscollected, 14770 independent [R(int)=0.0729], which were used in allcalculations. R₁=0.0518, wR₂=0.0968 for observed unique reflections[F²>2σ(F²)] and R₁=0.1018, wR2=0.1077 for all unique reflections. Max.and min. residual electron densities 2.353 and −1.423 eÅ⁻³.

W₂(TMhpp)₄, 3.

A sample of 0.120 g (0.098 mmol) of dark green, solid W₂(TMhpp)₄Cl₂,that had been freshly washed with hexanes and then dried under vacuumwas placed in a solid addition tube attached to a flask equipped with astir bar and 0.30 g of freshly cut and cleaned potassium metal in 15 mLof THF. An extremely dry glass frit joined with a side-arm tube servedas a cap to the reaction flask. After the solid was added to the flask,the mixture was degassed three times by the freeze pump-thaw method andthen heated using a gentle reflux (around 80-85° C.). After 30 min thedark green reaction mixture changed to brown and after 1 h to red-blood.After 2 h of reflux, the reaction mixture was cooled to room temperatureand half of the THF was removed under vacuum. The mixture was filteredthrough the attached fitted glass into a side-armed tube and brought todryness under vacuum producing a deep-red, solid. Isolated yield 0.081g, 72%. The solid was dissolved in 10 mL of toluene, set in a Schlenktube under nitrogen with a glass cap protected with high vacuum greaseand wrapped with Parafilm; the stopcocks and all joints were alsowrapped with Parafilm and a septum was fitted to the side arm; the tubewas placed in a freezer at −30° C. After two weeks, very small dark-red,block-shaped crystals suitable for X-ray diffraction were collected. Theproduct was stored in an ampoule under argon. ¹H NMR: (C₆D₆, 300 MHz,25° C.): δ_(H)=3.26 ppm (s, 16H, 8CH₂), 2.67 ppm (s, 16H, 8CH₂), and1.15 ppm (s, 48H, 16CH₃). PES_(onset): 3.74±0.03 eV. CrystallographicData for 3: M_(r)=1144.90, triclinic, P₁ ⁻, a=10.197(6), b=12.777(8),c=13.452(8) Å, α=112.441(9), β=90.278(10), γ=110.086(10)°, V=1502.6(16)Å³, Z=1, ρ_(c)=1.265 Mg m⁻³, T=213(2) K, λ=0.71073 Å. 9272 reflectionscollected, 5169 independent [R(int)=0.0871], which were used in allcalculations. R₁=0.0862, wR₂=0.1562 for observed unique reflections[F²>2σ(F²)] and R₁=0.1541, wR2=0.1744 for all unique reflections. Max.and min. residual electron densities 5.483 and −2.426 eÅ⁻³.

W₂(TEhpp)₄, 4.

This compound was prepared similarly to 3 using 0.14 g, 0.097 mmol ofdark green solid W₂(TEhpp)₄Cl₂, 0.3 g of clean potassium metal and 15 mLof THF. After refluxing and filtration of the deep red reaction mixture,the solvent was removed to produce 0.095 g of a deep red solid. Isolatedyield 0.095 g, 71%. The product was stored in a sealed ampoule underargon. ¹H NMR: (C₆D₆, 300 MHz, 25° C.): δ_(H)=3.48 ppm (s, 16H, 8CH₂),2.73 ppm (s, 16H, 8CH₂), 1.70 ppm (q, 16H, 8CH₂), 1.46 ppm (q, 16H,8CH₂) and 0.92 ppm (s, 48H, 16CH₃). PES_(vertical): 3.71±0.03 eV.

Photoelectron Spectroscopy (PES).

The gas-phase PES of W₂(TMhpp)₄ and W₂(TEhpp)₄ were recorded using aninstrument that features a 36-cm radius, 8-cm gap hemisphericalanalyzer, and custom-designed excitation source, sample cells, detectionand control electronics, and methods that have been described, forexample, in Rev. Sci. Instrum. 1986, 57, 2366 and J. Am. Chem. Soc.1998, 120, 3382-3386. The temperature was monitored using a “K”-typethermocouple passed through a vacuum feed through and attached directlyto the sample cell. Samples of W₂(TMhpp)₄ and W₂(TEhpp)₄ were loadedinto stainless steel cells and placed in the instrument using rigorousair-sensitive techniques. The data collection focused on determining theδ bond ionization energy. To avoid decomposition in the sample chamberof the cell, ampoules containing the samples were broken off and placeddirectly in the ionization chamber.

The W₂(TMhpp)₄ sample began to sublime at 216° C. and data was collectedcontinually with gradually increasing temperature until 271° C. when theloaded sample was fully consumed. Complete spectra were collected everyfew minutes. The sample lasted for ca. 2½ h in the chamber withoutevidence of decomposition. The individual spectral scans showed the sameionization features, and the displayed spectra are the sum of theindividual scans. Sublimation of the W₂(TEhpp)₄ sample began at around317° C. and was still present at 340° C. The signal was observed forabout 1 h again without evidence of decomposition, and the displayedspectrum is the sum of the individual scans.

The ²P_(3/2) peak of argon at 15.76 eV ionization energy is typicallyused for internal energy calibration, but over the course of theexperiment sensitivity to the ²P_(3/2) peak of Ar was lost. This oftenhappens with molecules that are strong reductants, presumably because ofthe very large change in work function of the spectrometer when thesample condenses on the spectrometer surfaces. At this point the lampwas adjusted to emit He II photons in addition to He I photons. Thisallowed observation of the He self-ionization (by He II photons) at anapparent binding energy of 4.99 eV in the He I spectrum. This very sharpline is a convenient internal energy calibrant and is preferable for thecalibration of low-energy ionizations. The resolution, as measured bythe Ar peak before it was no longer visible, was approximately 0.030 eV.The resolution for the W₂(TMhpp)₄ and W₂(TEhpp)₄ experiments, asmeasured by the He self-ionization, was 71 meV and 88 meV, respectively.Over the course of two separate data collections of W₂(TMhpp)₄,approximately 800 counts of the full He I spectrum (Graphic S1) and anadditional 400 counts of a close-up on the δ bond ionization werecollected. For W₂(TEhpp)₄ approximately 400 counts of the δ bondionization were collected.

The ionization bands are represented analytically with asymmetricGaussian peaks. As is typical for the first ionization bands of M₂(hpp)₄molecules, a weak shoulder is observed on the high ionization energyside of the peak that necessitates the use of a second asymmetricGaussian component to reasonably account for the total contour of theionization intensity. The additional ionization intensity on the highionization energy side of the first band is ascribed to the differentpuckered conformations that the hpp ligands can adopt in the molecules.The relative intensity of this component varies among the differentM₂hpp₄ molecules. The broadening on the high ionization energy side ofthe W₂(TEhpp)₄ ionization is the greatest, where additionalconformations due to the ethyl group orientations are possible. Thevertical ionization energies are reported as the position of the mainGaussian peak in the analytical representation. The onset of ionizationis the position at which the observed electron counts are significantlyabove a linear baseline through the peak.

Crystallographic data for 1, 2 and 3.

Crystals were coated with Paratone oil and mounted on a nylon Cryoloopaffixed to a goniometer head. Data were collected at 213 K on a BrukerSMART 1000 CCD area detector system using omega scans of 0.3 deg/frame,with exposures of 60 (1), 50 (2) and 80 (3) seconds per frame, such that1271 frames were collected for a full hemisphere of data. For eachcompound, the first 50 frames were collected again at the end of thedata collection to monitor for crystal decay. No significantdecomposition was observed. Cell parameters were determined using theprogram SMART. Data reduction and integration were performed with thesoftware package SAINT, which corrects for Lorentz and polarizationeffects, while absorption corrections were applied by using the programSADABS.

The structures were initially determined using the program XPREP fromthe SHELX software package. Compound 1 was identified as belonging tothe orthorhombic space group Pcca by its systematic absences. Because ofresidual disorder in solvent molecules, the Squeeze tool from Platonsoftware was applied to finish the structure refinement. The structurecontains two independent molecules in the asymmetric unit which meansthere are eight molecules per unit cell. Compound 2 gave a goodrefinement in the monoclinic space group P2₁/c. For this compound nointerstitial solvent was found; there is one molecule in the asymmetricunit and four in the unit cell. Compound 3 was refined, following theMarsh's recommendation of choose the higher symmetry group, in thetriclinic space group P₁ ⁻ instead of the non-centrosymmetric P1 thatwas the first choice suggested by the XPREP program; there is only onemolecule in the asymmetric and unit cell. The methyl groups from theTMhpp ligands were disordered where the major components havingoccupancies between 50.3 and 54.2%. The structure contained interstitialdisordered solvent that was treated using the Squeeze tool from thePlaton software.

Computational Model.

Several density functional and basis set models were tested for theirability to account for the geometric structures and first gas-phaseionization energies of this class of ditungsten complexes. As pointedout in the discussion these features are the most important in relationto the reduction chemistry of these complexes. Functionals were testedat the level of the local density approximation (LDA), the generalizedgradient approximation (GGA) with and without dispersion corrections,meta-GGA, hybrid, and meta-hybrid. Selected results with the AmsterdamDensity Functional program version adf2013.01 are shown in the SI. Allfunctionals overestimated the W—W bond length by about 0.05 Å andunderestimated the first ionization energy by as much as 0.6 eV. The LDAand GGA functionals had difficulty modeling the length of the weak W—Clbonds, and the breaking of these bonds with reduction is important inthe synthesis depicted in Scheme 1. Inclusion of dispersion with BJdamping with the PBE functional (PBE-D3) gave geometries that comparedgenerally as well with the experimental W—Cl distance as the familiarmeta-GGA and hybrid functionals, and gave a somewhat better firstionization energy at much less computational cost, and therefore thismodel was selected for the remaining computations. The basis setselected for the geometry optimizations and electronic energies wasdouble-zeta valence for hydrogen (DZ) and triple-zeta valence pluspolarization (TZP, with non-valence core) for all other atoms buttungsten. For tungsten an additional polarization function was added(TZ2P) to minimize basis set superposition errors (BSSE) for Cl⁻dissociation with reduction, but as shown below BSSE is not a concernfor this study because of the modeling approach. Relativistic effectsare included by the Zero Order Regular Approximation (ZORA). Solvationfree energies are estimated by the Conductor like Screening Model(COSMO) of solvation using default parameters.

Zero-point vibrational energies and thermal contributions to the freeenergy are computed at a lower level of theory because these terms havesmall contributions to the reduction chemistry for these complexes, andto save computational costs. The functional was the Vosko-Wilk-NusairLDA with Stoll's correction and the basis set was TZP for tungsten andDZ for all other atoms. Harmonic vibrational frequencies were calculatedanalytically and used without scaling to calculate the gas phasezero-point energies and thermal vibrational enthalpies and entropies. Itis known that the gas-phase translational and rotational entropiesoverestimate the entropies in solution, and this is especially a problemwhen the number of reactant molecules is different from the number ofproduct molecules. The gas-phase translational and rotational entropieswere scaled by 0.5 in solution similar to other adjustments in theliterature. This uncertainty was further mitigated by explicitlyincluding solvent molecules to balance the number of reactant andproduct molecules. For example, for the release of the Cl⁻ ion withreduction, the vacated W₂ axial coordination site was made to beoccupied with a THF molecule. This had the additional advantages of 1)the basis set superposition error of a vacant site was eliminated, and2) the explicit energy interaction of the THF molecule in the innersphere in combination with the continuum solvation model for the outersphere gave a better determination of the solvation energy.

The foregoing discussion of the invention has been presented forpurposes of illustration and description. The foregoing is not intendedto limit the invention to the form or forms disclosed herein. Althoughthe description of the invention has included description of one or moreembodiments and certain variations and modifications, other variationsand modifications are within the scope of the invention, e.g., as may bewithin the skill and knowledge of those in the art, after understandingthe present disclosure. It is intended to obtain rights which includealternative embodiments to the extent permitted, including alternate,interchangeable and/or equivalent structures, functions, ranges or stepsto those claimed, whether or not such alternate, interchangeable and/orequivalent structures, functions, ranges or steps are disclosed herein,and without intending to publicly dedicate any patentable subjectmatter. All references cited herein are incorporated by reference intheir entirety.

What is claimed is:
 1. A thin film comprising a compound of the formula:M_(a)(L)_(b)X_(c), wherein each M is independently tungsten or chromium;each L is independently a bicyclic heterocyclic anionic compound; X is ahalide; a is 1 or 2; b is an integer from 1 to 4; and c is an integerfrom 0 to 2, and wherein said solid crystal has an onset ionizationenergy of about 4 eV or less and is soluble in an organic solvent. 2.The thin film of claim 1, wherein said compound is soluble in an aproticorganic solvent.
 3. The thin film of claim 1, wherein said compound issoluble in a nonpolar organic solvent.
 4. The thin film of claim 1,wherein said compound has an oxidation potential in THF of about −1.5 Vor less.
 5. The thin film of claim 4, wherein said compound has anoxidation potential in THF of about −1.8 V or less.
 6. The thin film ofclaim 1, wherein said compound has an onset ionization energy of about3.6 eV or less.
 7. The substrate of claim 1, wherein the solubility ofsaid compound in THF is at least 0.1 g/L.
 8. The thin film of claim 7,wherein the solubility of said compound in THF is at least 1 g/L.
 9. Thethin film of claim 1, wherein the solubility of said compound in benzeneis at least 0.1 g/L.
 10. The thin film of claim 7, wherein thesolubility of said compound in benzene is at least 1 g/L.
 11. Anintegrated circuit comprising a compound of the formula:M_(a)(L)_(b)X_(c), wherein each M is independently tungsten or chromium;each L is independently a bicyclic heterocyclic anionic compound; X is ahalide; a is 1 or 2; b is an integer from 1 to 4; and c is an integerfrom 0 to 2, and wherein said compound has an oxidation potential in THFof about −1.5 V or less, and is soluble in an organic solvent.
 12. Theintegrated circuit of claim 11, wherein said compound has an oxidationpotential in THF of about −1.8 V or less.
 13. The integrated circuit ofclaim 11, wherein said compound has an onset ionization energy of about4 eV or less.
 14. The integrated circuit of claim 13, wherein saidcompound has an onset ionization energy of about 3.6 eV or less.
 15. Acompound of the formula:Ma(L)bXc wherein each M is independently tungsten or chromium; each L isindependently a bicyclic heterocyclic anionic compound; X is a halide; ais 1 or 2; b is an integer from 1 to 4; and c is an integer from 0 to 2.16. The compound of claim 15, wherein said bicyclic heterocyclic anioniccompound is of the formula:

wherein each of m and n is independently an integer from 0 to 4,provided the sum of m and n is at least 1; and each of R¹ and R² isindependently alkyl.
 17. The compound of claim 16, wherein R¹ and R² areindependently methyl, or ethyl.
 18. The compound of claim 16, wherein mis
 2. 19. The compound of claim 16, wherein n is
 2. 20. The compound ofclaim 16, wherein said compound is of the formula:M₂(L)₄Xc wherein M and X are those defined in claim 15; L is thatdefined in claim 16; and c is an integer of 0 or 2.